The present invention relates generally to a photographic projector-printer assembly and, more particularly, to a photometric unit for providing photometric determination of the density and spectral distributions, etc. of negative film used with such a photographic projector-printer assembly.
In a photographic projector-printer assembly for projecting negative film held by a negative holder onto a photographic paper for photographic printing, the density and spectral distributions of the negative film, the absorption spectra of the base film, etc. are photometrically determined. So far, this has been done by inserting a reflecting mirror in front of (or on the object side of) the projecting lens to guide the light from the negative film to an area sensor and removing the reflecting mirror from the optical path during printing. Photometry has also been carried out with an area sensor located out of and diagonally above the optical path. When the negative film used is varied in size, the area to be photometrically determined is also changed, correspondingly. In the latter case, there is particular need of changing the focal length of the lens through which an image on the negative film is formed on the light-receiving surface of the area sensor. One conventional photometric unit is so constructed by using a single area sensor in combination with a single zoom lens that the focal length of the zoom lens can be varied whenever the negative film is varied in size, thereby changing the area to be photometrically determined. Another available photometric unit is made up of a single area sensor and a plurality of image-formation lenses with different variable focal lengths.
However, problems with such units using a single zoom lens or a plurality of image-formation lenses with variable focal lengths are that they do not only have a complicated structure and a large size due to including some movable portion, but there is also some time lag in changing the focal lengths.
Such problems might potentially be solved, if two photometric systems, each comprising a combination of a single area sensor with a single image-formation lens having a fixed focal length, are located off and diagonally above the optical path and either one of them is selectively used whenever negative film is varied in size. However, this solution would pose an assembly problem and need much more space, because the optical axes of the photometric systems must be separately located in alignment with the center of the negative film.